The present invention relates to lubricating systems for shaft bearings, and more particularly to improvements in lubricating systems for bearings which are subjected to pronounced thermal stresses. Typical examples of such bearings are thrust bearings for the shafts of boiler feed pumps in nuclear reactor plants.
In accordance with the presently prevailing practice, shaft bearings which are maintained at elevated temperatures are lubricated by oil which is cooled in a heat exchanger. As a rule, the coolant is water. Such lubricating systems are satisfactory when the temperature in the region of the shaft bearing is below 100.degree. C. However, a boiler feed pump which is installed in a nuclear reactor plant must be constructed and assembled in such a way that, in the event of a catastrophy, its satisfactory operation remains assured at temperatures well above 100.degree. C. and when the relative moisture content of air is 100%. Moreover, such pumps must be capable of operating properly without any aqueous coolant for the lubricant. In many presently known safety boiler feed pumps for use in nuclear reactor plants, the shaft of a multistage centrifugal boiler feed pump with an average output is mounted in conical roller bearings which is rather unusual for such types of pumps. Conical roller bearings are quite satisfactory as long as the temperature in the surrounding area does not exceed 200.degree. C., i.e., the cooling action upon lubricant which is used for such bearings must suffice to ensure that the temperature of the bearing does not rise above 200.degree. C. The maximum permissible temperature for normal oils is in the range of 150.degree. C. but such temperature can be raised to approximately 200.degree. C. without cracking of oil by resorting to special additives. In order to prevent excessive heating of oil which lubricates a conical roller bearing, centrifugal pumps of the above outlined character are often equipped with gear pumps which circulate the oil and with specially designed oil cooling units. This contributes significantly to the initial and maintenance costs of boiler feed pumps.